1. Field of the Invention
The present invention relates to an image display device and a manufacturing method therefor. More particularly, the present invention relates to an image display device with a built-in driver circuit which includes a driver circuit on a substrate, and a method of manufacturing the image display device.
2. Description of the Related Art
There has been known a type of liquid crystal display device which uses a thin film transistor (hereinafter may simply be referred to as TFT) with a semiconductor film made of amorphous silicon, especially a bottom gate TFT which includes a gate electrode provided on a substrate side with respect to the semiconductor film (underside of the semiconductor film), as a switching element for driving a pixel (a TFT as this may hereinafter be referred to as pixel TFT).
Channel etch TFTs are known as one of the above-mentioned bottom gate TFTs. A channel etch TFT is formed by: forming a layered structure through sequential patterning of a gate, a gate insulator film, a semiconductor film, and a contact layer; forming a metal film in a manner that covers the sequentially layered structure; processing the metal film to form a source and a drain; and etching portions of the contact layer that are not covered with the source and drain electrodes, along with part of the semiconductor film formed below the etched contact layer.
In a TFT having this structure, an influence of an electric field from the gate electrode hardly reaches a front side of the semiconductor film (herein, a top side of a semiconductor film is the front side), whereas the influence of an electric field from above often reaches the front side of the semiconductor film, resulting in a problem that leak current occurs on the front side of the semiconductor film.
An example of solutions to this inconvenience is disclosed in JP 2002-305306 A, in which the front side of the semiconductor film is made porous and accordingly low in mobility, to thereby reduce leak current due to an external electric field. Other known methods involving forming a low-mobility layer than the ion irradiation method described in JP 2002-305306 A include one disclosed in JP 2003-37270 A which uses oxygen plasma and hydrogen plasma.
JP 2007-95190 A discloses a liquid crystal display device having a built-in driver circuit with a TFT using an amorphous Si semiconductor film. In JP 2007-95190 A and in JP 2002-175053 A, a dual gate TFT which has gates above and below the semiconductor film constitutes a part of the driver circuit of the liquid crystal display device.
In the above-mentioned image display devices, forming a circuit that drives a gate of a pixel with an amorphous silicon TFT (a TFT for this use is hereinafter referred to as gate driver TFT) makes the gate driver TFT, which writes in a gate line, large in size and increases a circuit width.
A TFT using a semiconductor film that is made of polycrystalline silicon, on the other hand, is superior in driving power and reduced in TFT size. However, manufacturing a polycrystalline silicon TFT requires processes that are not employed in a manufacture line for an amorphous silicon TFT, such as crystallization, impurity implantation, and activation.
The driving power of a gate driver TFT can be improved by forming a top gate above the semiconductor film of the gate driver TFT with a second gate insulator film interposed therebetween, thus giving the gate driver TFT a dual gate TFT structure in which a channel is formed on the front side of the semiconductor film as well. This is because the mobility on the front side of the semiconductor film is desirably high in order to improve the driving power. On the other hand, low mobility on the front side of the semiconductor film is desirable in a TFT that is used as a pixel switch and needs to suppress leak current.
In the conventional liquid crystal display devices, no consideration has been given to differentiate the mobility on the front side of the semiconductor film of a pixel TFT from the mobility on the front side of the semiconductor film of a gate driver TFT by, for example, employing different processes. Consequently, combining different characteristics required of the two TFTs has been difficult.